Lifting and moving large flat plates, such as manhole covers, is not only physically difficult, but also hazards the risk of injury to those involved. A typical manhole cover is anywhere from two to three feet in diameter, is about one inch thick, and more importantly, weighs in the range of 180 to 360 pounds. Their awkward size and weight has caused many a serious injury when only a pinch or pry bar is used to initiate lifting, with the final cover removal being produced by manual labor. An example of an improved pry bar intended to ease the initial lifting is shown in U.S. Pat. No. 3,837,622 by Gale.
A variety of configurations have received attention over the years to simplify plate handling. Some require special adaptation of the cover to match the tool, such as the threaded engagement devices of U.S. Pat. No. 3,198,362 by Berg, U.S. Pat. No. 4,157,810 by Haller et al, and U.S. Pat. No. 4,365,925 by Girtz. Such devices are useless for covers not modified to mate the engaging structure. In addition, dirt and debris from normal usage of the covers impedes use of the device, even if the cover is suitably modified.
Others have suggested use of L-shaped arms for insertion into holes in the covers, which arms are then forced or pivoted under the lower surface of the plate for lifting. Examples are shown in U.S. Pat. No. 4,126,297 by Barnes, U.S. Pat. No. 4,488,706 by Kono, U.S. Pat. No. 4,653,728 by Mochizuki et al and U.S. Pat. No. 4,662,607 by Mochizuki et al. The necessarily small size of the transverse lip of the L-shaped member, along with hazards resulting from wear of that lip during use, seriously discount these devices as adequate solutions. In addition, they too suffer from problems of debris plugging the holes and, of course, are useless for covers that have no holes. U.S. Pat. No. 3,275,299 by Meshew, although another version of the L-shaped arm approach, additionally suggests configuring a lower ground engaging cam surface to aid in raising the cover after it is engaged.
Another device for engaging manholes which have circumferential notches is shown in U.S. Pat. No. 4,482,182 by Mortensen, which device has hooks on the ends of both a parallel beam assembly and a handle that are interconnected by a sliding bar. The hooks are inserted into the edge notches and the handle is moved until a latch sets, securing the parallel beams and handle in place so that the user can lift the plate. Other than providing a convenient handle to lift the cover, Mortensen does not suggest any means for safely manipulating the cover once it is lifted.
Several arrangements of tools suitable for safely moving large flat plates and grates, using an approach wherein the plate is securely gripped at or near one edge of the plate, are disclosed in my co-pending patent application Ser. No. 07/156,380 filed Feb. 16, 1988 and entitled Maneuvering Device for Heavy Flat Plates.